How to increase throughput in amplifier testing
In the wireless industry, the notion of performing several time-consuming tests at a higher total cost is contrary to typical business imperatives: get devices to market quickly while managing prices, reducing costs, and preserving profit and performance margins. As a result, engineers who test mobile power amps and front-end modules on the production line can benefit from solutions that maximize throughput while ensuring product quality through sufficient testing of key performance requirements.
Within a test station, PXIe-based vector signal generators (VSGs) can provide increased test speed and throughput with capabilities such as quick frequency and amplitude switching and good signal linearity and repeatability. This article provides an overview of the key issues related to the testing of power amplifiers and front-end modules with an RF signal generator. It then presents a suggested test configuration and describes a range of throughput-enhancing capabilities provided by some new modular VSGs.
Key issues for power amp testing
Power amplifiers present two key test challenges: (1) assessing modulation performance at relatively high power levels and (2) quickly adjusting power levels into the device under test (DUT). Typical power amplifier modules require an input level of 0 to +5 dBm, digitally modulated according to communication standards such as W-CDMA or LTE. The specified performance characteristics of the power amp or front-end module are normally associated with a specific output level.
Required measurements include modulation quality and output spectrum. Modulation quality measurements are required to ensure the correct operation of the mobile device and spectrum measurements ensure that the device will not cause interference with other devices operating in an adjacent frequency channel. Error vector magnitude (EVM) is the most common measurement of modulation quality and adjacent-channel power ratio (ACPR) or adjacent-channel leakage ratio (ACLR) is the most common output spectrum measurement.
If a device has small variations in gain, it may be necessary to adjust the power level from the VSG to produce the correct output level from the DUT. Only after the DUT output level is set to the correct value can the specified parameters be tested. The time spent adjusting the VSG to get the correct DUT output power can be a major contributor to total test time and the overall cost of test.
Suggested test setup
The VSG is connected to the DUT using cables and switches (Figure 1). The switching may be used to support testing of multi-band modules, and the complexity of the switching depends on the number of bands in the devices. The DUTs are typically inserted into the test fixture using an automated part handler.
In some cases, several feet of cable is required between the VSG and the input of the DUT. The combined effect of cabling and switching can add several decibels of loss between the output of the VSG and the input of the DUT. Overcoming that loss without adding an external amplifier to the test system requires high output levels directly from the VSG.
Because most tests are performed with a modulated signal, the VSG must also provide adequate modulation performance at high power levels. The two most important measures of modulation quality for the VSG—and the amp-under-test—are ACPR and EVM.
To satisfy power amp testing requirement, the VSG must provide the following capabilities:
• Fast switching of both amplitude and frequency
• Very linear changes when adjusting the output power level
• Good modulation performance, particularly at high power levels
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